Fluid catalytic cracking (FCC) is one of the most widely used refinery processes for converting heavy oils into more valuable gasoline and lighter products. Wherever there is a rapidly growing demand for gasoline, FCC is the cheapest and fastest route to obtain this premium priced product. Consequently, much work has been done through the years to improve the yeild and/or octane rating of the FCC product slate. Paramount in improving the FCC yield and product slate are the cracking catalysts employed. While commercial cracking catalysts include acid-treated natural aluminosilicates, amorphous synthetic silica-alumina combinations, and crystalline synthetic silica-alumina (zeolites), the most widely used commercial fluid catalytic cracking catalysts are the zeolites. While zeolites for FCC have met with a very high degree of success, they are nevertheless limited by their respective pore sizes as to which hydrocarbon molecules can reach the active acid sites, the 7.4 Angstrom pore mouth of Y-faujasite being typical of the upper end of this pore size restriction. Furthermore, there is a tie between a zeolite of a particular structure and the aluminum content range of that zeolite. In certain limited cases, it is not possible to obtain a particular aluminum content in a specific zeolite structure. Even where this is not the case, it is not uncommon that one must employ chemical dealumination schemes subsequent to synthesis to obtain the specific aluminum content or other property that one desires. As a result, considerable effort has been expended to develop catalysts comprising either a naturally occurring or synthetic zeolite having, firstly, the desired cracking activity; secondly, pore sizes which will permit access to the acid sites of those hydrocarbon molecules sought to be cracked, and; thirdly, an expanded aluminum content range. To date, research has been unable to increase pore sizes beyond the 7-8 Angstrom range, and inroads into expanding the aluminum content range have been limited.
Generally, catalysts of this type are used in compositions frequently containing an "inert" matrix which will reduce cracking activity to a controlled level and which produces useful products; e.g., transportation fuels from the FCC process. Catalyst compositions of this type are taught in U.S. Pat. No. 4,289,606 and the patents and other references therein cited.
A further limitation is encountered in the use of composite catalysts in the aforementioned FCC process relating to by-product coke deposited on the catalyst. The catalyst becomes deactivated by the deposition of coke, and must be reactivated by burning the coke off the catalyst. The heat produced by burning is useful in that this is used to bring the feed to process temperature, and it counteracts the innate endothermicity of cracking reactions. However, it is generally the case that more coke is produced than is needed to heat balance the process, and since heat balance is the controlling parameter, catalyst is necessarily recycled in a partially deactivated form; i.e., containing residual coke. This situation is exacerbated by the progressively heavier nature of today's FCC feeds which are prone to making higher levels of coke. Thus, the need for a catalyst which makes less coke is clear. In addition, there is the need for catalysts which are more stable to the harsh conditions found in the regenerator (600.degree. C.-800.degree. C. under steam partial pressure). The steam changes the catalyst in a number of ways, perhaps the most important of which is to remove aluminum from the zeolite, thereby deactivating the zeolite by reducing the number of acid sites. Manifestly, there is also a need for catalysts with higher steam stability.
More recently, it has been discovered that synthetic, amorphous silica-alumina compounds, having the capability to catalyze conversion of oxygen-containing hydrocarbons, such as methanol to aromatic hydrocarbons such as toluene, can be prepared. Such a catalyst composition is taught in published U.K. Application No. 2,132,595A. This particular catalyst does not, however, exhibit significant cracking activity. Even more recently, it has also been discovered that the number of acidic sites in a crystalline silica-alumina catalyst, such as a synthetic zeolite, can be increased by coating the zeolite with alumina. Catalysts of this type are taught in published European Patent Application No. 0,184,305. Such catalysts, however, do not afford an opportunity to control cracking activity over a broad range of acid sites since the minimum activity is controlled by the activity of the zeolite initially selected. Furthermore, the maximum activity is quickly reached as the importance of the number of sites becomes outweighed by the lower activity of each site which results from closer site proximity. Moreover, the cracking activity of catalyst of the type taught in European Patent Application 0,184,305 is, to a large extent, controlled by the relatively small pore size of the ZSM- 5 zeolite initially selected for coating with alumina.
From the foregoing, and as is believed well known in the prior art, synthetic zeolite catalysts in composite with inert matrices have a controlled activity level that produces the desired product slate(s) in conventional commercial processes, but they suffer four drawbacks. Firstly, they are restricted as to the molecular weight of hydrocarbon compounds which can be cracked owing to the pore size thereof. Secondly, there are crystalline constraints on elemental composition; i.e., the aluminum content range which is available with a zeolite of a specific structure is limited, and a specific aluminum content can sometimes be achieved only subsequent to synthesis by a separate chemical dealumination step. Thirdly, incompletely regenerated catalysts must necessarily be recycled in processes such as FCC because the amount of coke made exceeds that which is needed to heat balance the process. Fourthly, zeolites are subject to structural degradation by reaction with high temperature steam which is a by-product of regeneration.